The invention relates to an oxygen-consuming electrode, especially for use in chloralkali electrolysis, with an improved catalyst coating, and to an electrolysis apparatus. The invention further relates to a production process for the oxygen-consuming electrode and to the use thereof in chloralkali electrolysis or fuel cell technology.
The invention proceeds from oxygen-consuming electrodes known per se, which take the form of gas diffusion electrodes and typically comprise an electrically conductive carrier and a gas diffusion layer and a catalytically active component.
Oxygen-consuming electrodes, also called OCEs for short hereinafter, are one form of gas diffusion electrodes. Gas diffusion electrodes are electrodes in which the three states of matter—solid, liquid and gaseous—are in contact with one another, and the solid electron-conducting catalyst catalyses an electrochemical reaction between the liquid and gaseous phases. The solid catalyst has usually been pressed to a porous film, typically having a thickness of more than 200 μm.
Various proposals for operation of the oxygen-consuming electrodes in electrolysis cells on the industrial scale are known in principle from the prior art. The basic idea is to replace the hydrogen-evolving cathode in the electrolysis (for example in chloralkali electrolysis) with the oxygen-consuming electrode (cathode). An overview of the possible cell designs and solutions can be found in the publication by Moussallem et al “Chlor-Alkali Electrolysis with Oxygen Depolarized Cathodes: History, Present Status and Future Prospects”, J. Appl. Electrochem. 38 (2008) 1177-1194.
The oxygen-consuming electrode—also called OCE for short hereinafter—must meet a series of fundamental requirements to be usable in industrial electrolysers. For instance, the catalyst and all other materials used must be chemically stable towards about 32% by weight sodium hydroxide solution and towards pure oxygen at a temperature of typically 80-90° C. Equally, a high degree of mechanical stability is required, in that the electrodes are incorporated and operated in electrolysers of a size of typically more than 2 m2 in area (technical size). Further properties are: a high electrical conductivity, a low layer thickness, a high internal surface area and a high electrochemical activity of the electrocatalyst. Suitable hydrophobic and hydrophilic pores and an appropriate pore structure for conduction of gas and electrolyte are needed, as is inperviosity, such that gas and liquid spaces remain separated from one another. Long-term stability and low production costs are further particular requirements on an industrially usable oxygen-consuming electrode.
A further development trend for utilization of OCE technology in chloralkali electrolysis is that of zero gap technology. In this case, the OCE is in direct contact with the ion exchanger membrane, which separates the anode space from the cathode space in the electrolysis cell. No gap containing sodium hydroxide solution is present here. This arrangement is typically also employed in fuel cell technology. A disadvantage here is that the sodium hydroxide solution which forms has to be passed through the OCE to the gas side and then flows downward along the OCE. In the course of this, the pores in the OCE must not be blocked by the sodium hydroxide solution, and sodium hydroxide must not crystallize in the pores. It has been found that very high sodium hydroxide solution concentrations can also arise here, but the ion exchanger membrane does not have long-term stability to these high concentrations (Lipp et al, J. Appl. Electrochem. 35 (2005)1015—Los Alamos National Laboratory “Peroxide formation during chlor-alkali electrolysis with carbon-based ODC”).
A conventional oxygen-consuming electrode consists typically of an electrically conductive carrier element to which the gas diffusion layer with a catalytically active component has been applied. The hydrophobic component used is generally polytetrafluoroethylene (PTFE), which additionally serves as a polymeric binder of the catalyst. In the case of electrodes with a silver catalyst, the silver serves as a hydrophilic component. In the case of carbon-supported catalysts, the carrier used is a carbon with hydrophilic pores, through which the liquid can be transported.
The catalyst used is generally a metal, a metal compound, a nonmetallic compound or a mixture of metal compounds or nonmetallic compounds. However, metals applied to a carbon support are also known, especially metals of the platinum group.
However, only platinum and silver have gained practical significance as a catalyst for the reduction of oxygen in alkaline solutions.
Platinum is used exclusively in supported form. A preferred support material is carbon. Carbon conducts electrical currents to the platinum catalyst. The pores in the carbon particles can be hydrophilized by oxidation of the surfaces to make them suitable for the transport of water.
Silver can also be used in accordance with the prior art with carbon as a support, and also in the form of finely divided metallic silver.
In the case of production of OCEs with unsupported silver catalyst, the silver can be introduced at least partly in the form of silver oxides, which are then reduced to metallic silver. The reduction is effected either in the initial phase of the electrolysis, in which conditions already exist for reduction of silver compounds, or in a separate step prior to commencement of operation of the electrode, by electrochemical, chemical or other means known to those skilled in the art. The size of the silver particles is in the range from greater than 0.1 μm to 100 μm; typical particle sizes are 1 μm to 20 μm.
The production of oxygen-consuming electrodes with unsupported silver catalysts can in principle be divided between dry and wet manufacturing processes.
In the dry processes, a mixture of catalyst and polymeric component is ground to fine particles. This precursor is subsequently distributed over the electrically conductive carrier element and pressed at room temperature. Such a process is described in EP 1728896 A2.
In the wet manufacturing processes, a precursor in the form of a paste or of a suspension comprising fine silver particles and a polymeric component is used. Suspension medium used is generally water, but it is also possible to use other liquids such as alcohols or mixtures thereof with water. In the production of the pastes or suspension, it is possible to add surface-active substances in order to increase the stability thereof. The pastes are applied to the carrier element by means of screen printing or calendering, while the less viscous suspensions are typically sprayed onto the carrier element. The paste or suspension is gently dried after the emulsifier has been rinsed out, and then sintered at temperatures in the region of the melting point of the polymer. Such a process is described, for example, in US20060175195 A1.
The in the above-described oxygen-consuming electrode with unsupported silver catalysts have a good long-term stability under the conditions of electrolysis of alkali metal chlorides.
An important prerequisite for the operation of gas diffusion electrodes is that both the liquid and gaseous phases may be present simultaneously in the pore system of the electrodes. The gas is transported in pores with hydrophobic surfaces. These are primarily the pores which are formed by the fluoropolymer matrix. Hydrophobic pores are also present in the carbon, unless they have been provided with a hydrophilic surface by specific treatment. The liquid is transported in the pores with hydrophilic surfaces. These are generally pores which are formed by the silver catalyst, but also pores in hydrophilized carbons, as used as a support for catalysts. The pores for gas transport preferably have a greater diameter than the pores for liquid transport. The latter fill up completely with liquid; the capillary forces counteract the gas pressure. If the gas pressure is too high, liquid is displaced from the hydrophobic pores; conversely, in the case of excessive pressure on the liquid side, hydrophobic pores are flooded. Due to the lower capillary action, liquid is displaced more easily from the larger hydrophobic pores than from small pores.
The OCE should be impervious at a pressure differential between the gas and liquid spaces in the range from 10 to 60 mbar. What is meant here by “impervious” is that the naked eye cannot observe escape of gas bubbles into the electrolyte space, and less than 10 g/(h*cm2) of liquid passes through the OCE (g=mass of liquid, h=hour and cm2=geometric electrode surface area). If, however, too much liquid passes through the OCE, it can only flow downward on the side facing the gas side. This can form a liquid film which hinders the access of gas to the OCE and thus has an extremely adverse effect on the performance of the OCE (oxygen undersupply). If too much gas enters the electrolyte space, the gas bubbles close off some of the electrode and membrane area, which leads to a shift in current density and hence, in galvanostatic operation of the cell, to a local increase in current density and to an unwanted increase in cell voltage over the cell.
The production of the electrodes by the known dry and wet processes gives rise to pores of different size and distribution. The size and distribution can be controlled by the selection and pretreatment of the polymers and of the catalysts, and the method of application to the carrier. It is also possible to add pore-forming components in the course of production of the oxygen-consuming electrode. A known example is the addition of ammonium carbonate powder to the mixture of catalyst and polymeric component, which is driven out by heating after compaction of the catalyst mixture.
The flow resistance in the pores is inversely proportional to the diameter thereof. The known processes for production of OCEs give rise to pore distributions which ensure sufficient mass transfer. However, there is always a proportion of pores which are too small or too large. While the small pores are not troublesome in any way, the large pores offer too low a resistance to gas flow, which readily results in unwanted passage of gas through the electrode.
It is also possible, in the pressing and sintering processes mentioned, for channels and cracks to occur in the gas diffusion layer, through which gas can again pass into the liquid phase. Such channels and cracks can also form in the course of operation of an oxygen-consuming electrode, which worsens the performance over the service life.
It is an object of the present invention to provide an oxygen-consuming electrode for oxygen reduction under alkaline conditions, for example for use in chloralkali electrolysis, which overcomes the above disadvantages and enables a lower operating voltage in chloralkali electrolysis.
It is a specific object of the invention to provide an oxygen-consuming electrode which has low gas perviosity than conventional electrodes.
It is a further object of the invention to provide a simple means for improving or repairing existing oxygen-consuming electrodes which have started to leak or whose performance is or has become inadequate.
The object is achieved by providing the oxygen-consuming electrodes (OCEs) known per se, on the side facing towards the liquid or the ion exchanger membrane, with a finely divided hydrophilic component, said finely divided hydrophilic component being applied to the oxygen-consuming electrode in the form of a suspension in a suspension medium removable by vaporization.